1. Field of the Invention
The present invention relates to an active material for an electrode that can be used for a positive active material of a non-aqueous secondary battery and a non-aqueous secondary battery whose cycle characteristics and storage characteristics at a high temperature are improved by using such an active material for a positive electrode.
2. Description of Related Art
In recent years, along with the development of portable electronic equipment such as mobile phones and notebook computers, and the commercialization of electric vehicles, there is an increasing demand for a miniaturized and lightweight secondary battery with a high capacity. At present, as a secondary battery with a high capacity satisfying this demand, a non-aqueous secondary battery using LiCoO2 as a positive electrode material and a carbon material as a negative active material is being commercialized. Such a non-aqueous secondary battery has a high energy density, and can be miniaturized and reduced in weight, so that it has been paid attention to as a power source of portable electronic equipment. Since LiCoO2 used as a positive electrode material of the non-aqueous secondary battery is easy to produce and handle, it is often used as a preferable active material. However, LiCoO2 is produced using Co, which is rare metal, as a material. Therefore, it is conceivable that a material shortage will become serious in the future. Furthermore, the price of Co is high and fluctuates greatly, so that it is desired to develop a positive electrode material that can be supplied stably at a low cost.
In view of the above, complex oxide materials of a lithium-manganese oxide type containing Mn, which can be supplied stably at a low cost, as a constituent element hold great promise. Among them, LiMn2O4 with a Spinel structure, which can be charged/discharged in a voltage range in the vicinity of 4 V against Li metal, and LiMnO2 with a layered structure are being investigated. In particular, lithium-containing complex oxides obtained by substituting a part of Mn of the above-noted LiMnO2 with Ni, Co, Al etc. are expected to be a prospective substitute for LiCoO2 (see paragraphs 0027 to 0029 of JP 8(1996)-37007 A, paragraphs 0003 to 0008 of JP 11(1999)-25957 A and paragraphs 0002 to 0009 of JP 2000-223122 A).
However, when the detailed study of this complex oxide obtained by substituting a part of Mn of the above-noted LiMnO2 with Ni, Co etc. was conducted by the inventors of the present invention, it was found that the properties such as the structure and characteristics are changed remarkably due to the composition of a compound, in particular, the ratio of quantity of Li to other metallic elements, the kind and quantity ratio of substitute elements, and the synthesis process in which the complex oxide is formed.
Especially in the case of Ni substitution, due to the quantity ratio of Mn to Ni and the quantity ratio of these elements to other substitute elements, the property of the complex oxide to be synthesized varies greatly. Thus, unless the quantity ratio of Mn to Ni is about 1:1 and that of Mn and Ni to other substitute elements is in a certain range, it was not possible to obtain a homogenous compound with an excellent property. Also, depending on the quantity ratio of Mn and other substitute elements to Li, the true density of the complex oxide varies greatly.
Moreover, the form of particles in the above-described lithium-containing complex oxide considerably influences battery characteristics.